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Ball Bond Aging with Integrated Under-Pad Microheater

Keywords: reliability, wire bond, microheater

The reliability of new wire bonding processes needs to be high enough to ensure that the bonds can survive in operating conditions. One way to assess reliability is by storing sample bonds at high temperatures and measuring the reduction of shear strength or pull strength over time. When aged in ovens, all chips are aged at the same oven temperature, the chips may not be observed while inside the oven, and high temperature cables are required if any signals are measured from the chips during aging.
A new method of aging wire bonds is presented. A microheater is integrated into a test chip which is fabricated using a standard CMOS process. An n-well underneath a large test pad acts as a heating resistor. In operation, it raises the temperature of test bonds up to 297 °C. A resistive temperature detector is located next to the test pad. Using this chip, a sample of test bonds is aged for 500 h at 200 °C, while contact resistance of a bond is measured, and growth of intermetallic compounds (IMCs) adjacent to the bonds is monitored periodically every 30 min, providing evidence of advancing age.
Advantages of the integrated heater include the ability to maintain different temperatures for each chip being aged, localized heating at the bond pads, which permits low temperature electrical connections to the chip for signal monitoring, and an exposed chip surface allowing for optical real-time observation of IMC growth around the bonds.